Poor light stability hinders the potential applications of perovskite optoelectronic devices. Recent experiments have demonstrated that the passivation surface via forming strong chemical bonds (SO4 -Pb, PO4 -Pb, Cl-Pb, O-Pb, and S-Pb) could effectively improve the light stability of perovskite solar cells. However, the underlying reasons are not clear. Herein, the elusive underlying mechanisms of light stability enhancement are explained in detail using first principles calculations. The small polaron model and self-trapped exciton model demonstrate that an iodine vacancy defect on the surface of perovskite could trap a free electron under light illumination, which leads to a significant rearrangement of the Pb-I lattice and creats a new chemical species, i.e., a Pb-Pb dimer bound in the typical perovskite of CH3 NH3 PbI3 . The Pb-Pb dimer distorts the Pb-I octahedral lattice and reduces the defect formation energy of the I atoms. The surface Pb site passivation can prevent the formation of the Pb-Pb dimer, thereby improving the light stability. In addition, the strong ionic bond could better stabilize the Pb site. The in-depth understanding of the light stability and the passivation mechanism in this study can promote the application of perovskite optoelectronic devices.
Keywords: Pb-Pb dimers; iodine vacancy; light stability; passivation; perovskites.
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